The invention lies in the field of the packaging industry and relates to a device according to the generic part of the first independent claim. The device serves for carrying out a plasma enhanced process in a plastic container with a narrow opening in order to treat the inside surface of the container. Furthermore, the invention relates to a method of a plasma enhanced treatment using the inventive device and to containers produced according to the process.
Plasma enhanced processes used for treating the inside surface of plastic containers are e.g. plasma enhanced chemical vapour deposition processes for producing an inside coating of e.g. silicon oxide which coating reduces gas permeability of the plastic material. Plasma enhanced processes can further be used e.g. for activating the inside surface, for changing its wetability or for sterilizing the inside of the container. The constraints put on such processes and on the devices for carrying out such processes when applied to the inside of plastic containers used for packaging purposes and having a narrow opening such as e.g. bottles for beverages, are caused mainly by the temperature sensitivity of the plastic material, by the narrowness of the opening, i.e. the difficult accessibility of the surface to be treated and by the necessary small economic value of such containers.
From the publication EP-299754 (BOC Group) it is known that by coating plastic materials with a layer of silicon oxide, the gas permeability of such materials can be reduced considerably. The same publication discloses a plasma enhanced chemical vapour deposition process for producing such silicon oxide coatings.
From the publications EP-668219 and EP-667302 (Tetra Pak) it is further known that silicon oxide coatings produced by plasma enhanced chemical vapour deposition and having a composition of SiOx, whereby x ranges between 1.8 and 2.2, have superior gas barrier properties and that such coatings with a thickness which is less than 0.2 xcexcm (2000 Angstrxc3x6m) provide a stopped here gas barrier which is satisfactory for most applications in food and beverage packaging.
It is known e.g. from the publication by J. Weichart, B. Meyer and J. Mxc3xcller (Vakuum in der Praxis Nr.1, pages 22 to 26, 1991) that a plasma excited by microwave exhibits a higher electron density and higher electron energy than a plasma excited by direct current or by high frequency. Therefore, using a microwave plasma results in higher deposition rates for chemical vapour deposition processes. In a microwave plasma positioned in a static magnetic field the electron loss through diffusion is reduced and energy take-up by electrons is increased. This effect is due to magnetic confinement in which electrons are accelerated on paths around magnetic field lines creating within the microwave plasma electron-cyclotron-resonance conditions. Such conditions are fulfilled for a microwave frequency of 2.45 GHz and a pressure of less than 10 Pa in a static magnetic field of a magnetic flux density of 87.5 mT. Under such conditions it is possible to sustain a plasma with a much smaller power input than needed for e.g. a high frequency plasma
In the same publication an apparatus for coating the inside surface of bottles is proposed in which apparatus the microwave field is produced by a helical Wanderfeld antenna and the bottle is placed within the helix. The magnetic field is produced by permanent magnets arranged on the inside of the bottle along a gas feed tube which extends along the bottle axis.
In the same publication it is stated that silicon oxide coatings produced with a microwave plasma at electron-cyclotron-resonance conditions in the apparatus as described do not show permanent good barrier properties due probably to great hardness and brittleness.
In a plurality of more recent publications (e.g. Polar materials U.S. Pat. No. 5,378,510, Coca-Cola Company WO-95/22413, Toyobo JP-07041579), the inside coating of bottles with silicon oxide has again been described. As plastic bottles with an inside coating of silicon oxide are not available on the market, it can be concluded that up to now it has not been possible to produce the bottles with a satisfactory quality and/or with an effort which can be kept within reasonable limits.
It is therefore the object of the invention to show a device and a method for treating the inside surface of plastic containers having a narrow opening (e.g. bottles with an opening of 28 to 38 mm diameter) in a plasma enhanced process, e.g. coat this inside surface with silicon oxide using a plasma enhanced chemical vapour deposition process. Using the inventive device is to make it possible to ignite and sustain on the inside of the container a plasma possibly under electron-cyclotron-resonance conditions such that the plasma enhanced treatment compared to known such treatments is improved regarding quality of the treatment (e.g. quality of the resulting coating) and at the same time regarding energy and time consumption.
This object is achieved by the device and the method as defined by the claims.
The inventive device has the form of a finger and is introduced into the container to be treated through its opening. For being able to be introduced, it has a diameter which is less than the diameter of the container opening. In the container it extends substantially along the container axis, its connecting end passing through the container opening and being connected to appliances outside of the container and its distal end facing the container bottom opposite the container opening.
The device comprises means for feeding the process gas as uniformly as possible to the inside of the container and means for establishing a static magnetic field with closed field lines within the container such that the loaded particles, in particular the electrons created in a plasma are induced to rotate between the area of the container opening and the area opposite the container opening. Thereby, the magnetic field is advantageously of such a magnitude that electron-cyclotron-resonance conditions are possible within the container. The device may further comprise cooling means for cooling the means for feeding the process gas and if applicable the means for establishing the magnetic field.
The device is to be connected to a source of a process gas for supplying the process gas to the gas feed means. If applicable, the device is further to be connected for supply and removal of a cooling medium. The connecting end of the device passing through the container opening is designed such that it occupies as little of the container opening as possible. This makes it possible to evacuate and vent the container very efficiently.
The inventive device is preferably used in connection with a plasma sustained by microwave or radio frequency electromagnetic waves, i.e. by electromagnetic waves of frequencies in the range of ca. 30 KHz to several GHz, whereby the electromagnetic waves are coupled into a confinement in which the container to be treated is positioned by suitable coupling methods and means (hollow conductor, capacitive coupling or inductive coupling).
A container is treated on its inside in a plasma enhanced process by e.g. introducing the container into a microwave confinement, which confinement is equipped such that an advantageously stationary microwave can be established within the confinement. The device as described above is introduced into the container, the connecting end passing through the container opening substantially axially. Then the inside and outside of the container is evacuated, whereby on the inside of the container a pressure is created which is low enough for igniting a plasma. Microwaves are coupled into the microwave confinement and the process gas is flown through the gas feed means into the inside of the container and is pumped from the container such that a low operation pressure is maintained. After the reaction time the microwave is stopped, the system vented, the inventive device removed from the container and the container removed from the microwave confinement.
In case of a multi-step plasma enhanced treatment the process gas is changed after each step.